This invention relates to a fuel cell system and more particularly to a system having a plurality of cells which consume an H2-rich gas to produce power for vehicle propulsion.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cells gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A group of cells with the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and admixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors such as a water/gas shift (WGS) and preferential oxidizer (PROX) reactors are used to produce carbon dioxide (CO2) from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,442 and 08/980,087, filed in November, 1997, and U.S. Ser. No. 09/187,125, filed in November, 1998, and each assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell system depends on the ability to effectively control the amount of oxygen provided to the reactors and to the fuel cell stack while maintaining the level of hydrogen supplied to the anode in the fuel cell stack. This is particularly difficult during transient operation of a vehicular fuel cell system wherein the reformate fuel requirements vary with the changing loads placed on the fuel cell.
Therefore, it is desirable to provide a method and apparatus by which a controlled amount of oxygen is supplied as a reactant to catalyze the oxidation of carbon monoxide while maintaining a high level of hydrogen in the anode fuel stream particularly during dynamic system operation.
The present invention is directed toward a method and apparatus for controllably supplying oxygen to promote the oxidation of carbon monoxide while avoiding excessive oxidation of hydrogen in a fuel cell reformate stream.
The fuel cell system of the present invention comprises a source of a reformate stream which contains hydrogen and carbon monoxide (CO). The reformate stream typically contains more hydrogen on a volume basis than CO. The fuel cell system further comprises a PROX reactor for selectively oxidizing the CO through contact of the stream with a catalyst inside the PROX reactor chamber. The catalyst is supported by a carrier within the chamber and the chamber includes an inlet and an outlet allowing the stream to pass through the reactor chamber and over the catalyst.
The fuel cell system further comprises an injector system which includes at least one electrically energized injector for supplying controlled pulses of a gaseous mixture containing oxygen, or preferably air, into the stream containing hydrogen and CO at a predetermined location or locations along the stream. In one location, the injector is placed downstream from the reformate source, but upstream from the PROX reactor. Alternate or complimentary locations are directly into the PROX reactor or downstream of the PROX reactor. The injector is electrically energized by a control unit which may also establish and regulate the duration of each pulse; and also regulate the frequency or time between the pulses wherein no oxygen is added to the stream.
The precise supply of an oxygen mixture in controlled pulses by the injector system advantageously provides a sufficient amount of oxygen to promote oxidation and thereby consumption of CO with minimal or lesser oxidation and consumption of hydrogen in the stream which is essential for efficient operation of the fuel cell and system. The advantageous result is a CO depleted reformate stream with high levels of hydrogen feeding the anode in the fuel cell. The fuel cell system further comprises one or more fuel cells downstream of the PROX reactor which receives and consumes the hydrogen-rich, CO depleted reformate stream to produce electrical energy.
The injection system of the present invention may also comprise a similar second injector between the PROX reactor and the fuel cell which functions to promote oxidation of remaining unreacted CO in the stream prior to the stream entering the anode inlet in the fuel cell. Thereby, the CO content of the stream is further reduced.
The present invention may also comprise devices for monitoring the amount of hydrogen in the reformate stream upstream and downstream of the reactor. A control unit may receive signals from the monitoring devices and compare the respective amounts of hydrogen upstream and downstream of the reactor. Based on the input, the control unit would adjust the pulses by the injector to reduce the level of CO while minimizing excessive consumption of hydrogen in the reformate stream.
The present invention may also comprise a device for monitoring the amount of CO in the stream downstream of the reactor. A control unit may receive a signal from the CO monitoring device and similarly adjust the pulses by the injector to maintain the desired levels of CO and hydrogen in the reformate stream. Thus, the invention enhances oxidation of CO to CO2, while minimizing excessive oxidation of H2 to H2O.
Advantageously, the present method and apparatus is adaptable to, and easily implemented in, existing fuel cell systems which comprise a fuel processor, preferential oxidizer (PROX) and stack of fuel cells. The present method can be implemented in existing process controllers. In addition, the present monitoring method and apparatus is useable within a variety of fuel cell system configurations.